docs/tgsi: do not use math-block for non-latex

This block isn't valid latex, so let's just use a pseudocode-block like
we do elsewhere here.

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21893>